1. Field of the Invention
The present invention relates to radios for use in vehicles, and, more particularly, to improving signal reception quality in radios for use in vehicles.
2. Description of the Related Art
Car radio reception quality is an important element of overall consumer vehicle satisfaction. Consequently, car original equipment manufacturers (OEMs) and suppliers perform extensive in-field testing in different countries to tweak the reception quality to suit each market segment.
Users listening to radios while driving near an AM or FM radio transmission tower may hear two types of distortion. A first type of distortion is front end overload distortion where transmission from a nearby station overwhelms the car radio's RF digital signal processing (DSP) receiver. Front end overload can lead to clipping distortion at the intermediate frequency analog-to-digital conversion chain process. A method to avoid this is to increase the attenuation at the front end and tweak the automatic gain control (AGC) that is prior to the analog-to-digital conversion (ADC) stage. However, since the overload affects the entire FM frequency range, the radio frequency (RF) designer is presented with the challenge of accommodating both strong and weak signal reception in such a scenario.
A second type of distortion is inter-modulation distortion arising from the use of non-linear devices. In the car radio environment, the non-linear devices are primarily the low noise amplifier (LNA) and the heterodyne mixer or mixers depending on whether the heterodyne mixing process is one step 10.7 MHz intermediate frequency or a lower frequency down-shifted base band intermediate frequency operating for low power devices using multi-stage down conversion. The current trend with respect to low power devices is to operate in base band intermediate frequency to ensure that the sampling rate is lower, as this translates into lower power utilization at the analog-to-digital conversion stage onwards.
Inter-modulation occurs when the input to a non-linear device (NLD) is composed of two or more frequencies of high signal levels and results in the creation of frequency artifacts that are a product of the inputs. These artifacts can result in either new ‘phantom’ stations (e.g., artifacts occur on frequencies where no valid station exists in the vicinity of the car radio) or overlap on a existing valid frequency. When a user tunes to the overlapped frequency, he hears audio modulations from the multiple audio sources including the valid radio station and the modulations arising from station frequencies involved in the creation of the artifacts themselves.
For example, third order inter-modulation can arise from the following permutations:L*f1+/−M*f2+/−N*f3, where f1, f2 and f3 are distinct frequencies and L+M+N=3, where L, M and N are integers  (1)Here f1, f2 and f3 are signals over 70 dBuV (which may be calibratable)OR fromL*f1+/−M*f2, where f1, f2 are distinct frequencies and L+M=3 where L and M are integers  (2)Here f1 and f2 are signals over 70 dBuV (which may be calibratable)
While inter-modulation is typically caused by a car's proximity to strong transmission towers, other causes may originate from inside the car's passenger compartment through the use of powerful in-car FM transmitters which are used to stream audio from an external device (e.g., an iPod or external mp3 player) into a non-receivable FM radio station frequency so that the external audio source can be heard through the car speakers. These devices may output signal levels from 70 to 90 dBuV. Signals of a level exceeding 70 dBuV are considered strong signals and when mixed with other strong signals in the vicinity of the car, can lead to third order inter-modulation artifacts.
While inter-modulation distortion in the car radio can be of second order and third order types, the third order inter-modulation poses a bigger problem than second order inter-modulation. This is because second order inter-modulation can be typically filtered out using the band-pass filter. However, third order inter-modulation is harder to filter out as it lies very close to the center frequency of the frequency tuned by the radio head unit. A filter with characteristics steep enough to filter out third order inter-modulation but leave the tuned frequency intact is difficult to achieve.
Illustrated FIG. 1 is an example of typical prior art RF receiver topology that results in the creation of inter-modulation artifacts. The RF signal from the antenna goes through a low noise amplifier (LNA), which is a non-linear device, and then goes through a band-pass filter which tends to filter out frequencies outside the FM band. The next stage is the mixing with the local oscillator to provide the intermediate frequency. The mixer is also a non-linear device. The output product from the mixer passes through another filter stage to ensure that only the required intermediate frequency is output before the signal is digitally sampled at the RF analog to digital converter (ADC) and then again passes through an intermediate frequency (IF) filter.
FIG. 2 illustrates the characterization or mapping of the input power versus output power of a typical non-linear device. The plotted line 10 represents the third order inter-modulation characterization. The gain of the output inter-modulation product is based on the slope of line 10. For Global A boards, for example, the third order inter-modulation is between 10 and 15 dBuV and is known to cause audio distortion.
Illustrated in FIG. 3 is an example expanded characterization of output power versus input power for a non-linear device. FIG. 3 illustrates a typical model that is used to characterize the level of artifacts created. Line 12 represents the third order inter-modulation characterization.
The level of expected inter-modulation is shown in FIG. 4, which illustrates modeling of third order inter-modulation. The third order input intercept point (IIP3) is in units dBm and is a function of ΔP from the input levels of the fundamental strong frequencies at the input to the non-linear device.
FIG. 5 illustrates the third order intercept point (IP3) inter-modulation power increase for non-linear devices with no saturation. As shown in FIG. 5, the effects of inter-modulation vary based on the RF design and the characteristics of the components used. If the system has no saturation, then the third order inter-modulation can be as high as the fundamental frequencies at the input of the non-linear device.
FIG. 6 also illustrates IP3 inter-modulation power increase for non-linear devices with no saturation. As shown in FIG. 6, the third order inter-modulation effects depend on the performance of the gain stages at the latter part of the RF chain. This is true because the gain value increases geometrically towards (Gn) at the end of the chain.
FIG. 7 illustrates a characterization of the problem posed by third order inter-modulation. FIG. 7 highlights the reason why it is difficult to filter out the third order inter-modulation artifacts. While the second order harmonics are outside the pass band, the third order inter-modulations such as 2f1-f2 and 2f2-f1 are very close to the fundamental frequencies f1 and f2 (where f1 and f2 are strong signals of 70 dBuV or above). Because of the difficulty in filtering the third order inter-modulations, this poses a serious reception problem.
Accordingly, what is neither anticipated nor obvious in view of the prior art is a method of sensing inter-modulation distortion and mitigating its effects on signal reception quality.